Phylum Platyhelminthes (The Flatworms)

Phylum Platyhelminthes:

  • Platyhelminthes is a phylum of dorsoventrally flattened acoelomate but triploblastic animals having bilateral symmetry and blind sac body plan.
  • Due to their flattened nature, Platyhelminthes are also called flatworms.
  • The term Platyhelminthes was coined by Gegenbaur (1859).

Characteristics of Phylum Platyhelminthes:

  • Flatworms are free-living commensal or parasitic organisms. The free-living worms may be terrestrial or freshwater but a majority of endoparasitic.
  • The body is dorsoventrally flattened, leaf-like or ribbon-like.
  • The parasitic worms bear suckers and hooks to get attached themselves with hosts.
  • Platyhelminthes are triploblastic animals with three germ layers- ectoderm, endoderm and mesoderm.
  • Spaces between the body organs are filled with a mesodermal connective tissue, called parenchyma.
  • Flatworms exhibit a definite organ system level of organisation. Flatworms are the first and the simplest animals that have organs and organ systems. The systems are simple.
  • Excretion and osmoregulation occur by peculiar cells called flame cells or protonephridia or solenocytes. These are ammonotelic.
  • The alimentary canal is branched or unbranched with an opening called a mouth.
  • The nervous system includes a pair of an anterior ganglion (brain ganglion) and a ventral nerve cord.
  • A skeletal system is absent. Flatworms are, therefore, soft. In certain cases scleroprotein cuticle is present.
  • Flatworms have definite anterior and posterior ends. The concentration of sense organs at the anterior end marks the beginning of cephalization.
  • Most of the flatworms are hermaphrodites but can reproduce asexually and by sexual methods. In sexually reproducing organisms, cross-fertilization occurs in most cases.
  • Fertilization is internal and development is indirect through many larval stages.
  • The power of regeneration is well marked in some flat worms.

Examples- Taenia Solium (Tapeworm), Fasciola (Liver Fluke), Echinococcus (Dog Tapeworm) and Planaria.

Advancement Over Coelenterates:

  • Formation of organs.
  • Cephalisation.
  • Three germ layers.
  • Polarized ladder-like a nervous system.
  • Gonads with gonoducts.
  • Copulatory organ.
  • Flame cells as excretory structures.
  • The occurrence of muscles in the body wall and gut wall.
  • Bilateral symmetry.

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